In the early hours of Christmas Day 1912 the Antarctic explorer and scientist Douglas Mawson was trekking across the endless plateau of the frozen continent with his companion, Xavier Mertz. They were hundreds of miles from base camp and, 10 days earlier, they had been struck by tragedy when the third member of their team, the British officer Belgrave Ninnis, had fallen into a crevasse with his sledge and died.

"We wished each other merry Christmases in the future," Mawson wrote in his diary on 25 December. "I found two bits of biscuit in my bag so we had a piece each. We started at 2.30am, did 10 miles on a course WNW (general), rising for about 3 miles."

They had lost most of their food along with Ninnis and, to stay alive, Mawson and Mertz had started eating their sledging dogs. At 9.30am on Christmas morning, exhausted from their overnight trek, they set up camp and heated up their meagre Christmas dinner. "An ounce each of butter was served out from our small stock to give a festive touch to the dog-stew," wrote Mawson in his account of the original Australasian Antarctic Expedition (AAE).

Adelie penguins nesting at Cape Denison in East Antarctica, near the site of Douglas Mawson's huts Photograph: Alok Jha/Guardian

Almost 50 people – half of them scientists, half of them paying members of the public who volunteer as assistants on the research projects – have been sailing through the Southern Ocean on board the Shokalskiy research ship, repeating and extending many of Mawson's wildlife and weather observations in order to build a picture of how this part of the world had changed in the past 100 years.

Mawson's records are one of the earliest and most valuable scientific datasets that exist about Antarctica. Despite the unfolding horror of his trek over the Christmas of 1912, he continued this work – even after Mertz died two weeks after Christmas; no one knew at the time that the dog livers they were eating contained toxic levels of vitamin A and Mertz succumbed to a fever, probably caused by the poisoning.

But the visit, a century on, to sites such as Mawson's "Penguin Hill" discovered that there had a been decline in numbers of Adelie penguins, a problem most likely due to changing ice patters in the region, a predominance of unbroken fast ice that means the local colony no longer had easy access to the open sea to feed.

Supplies being unloaded on the original Australasian Antarctic Expedition. Photograph: Frank Hurley/State Library of NSW

Direct access from the sea has been impossible for the past four years, however, ever since a 75-mile-long iceberg called B09B grounded itself in the entrance to Commonwealth Bay. A thick band of sea ice has since built up around the iceberg, sticking fast to the land and blocking ships from getting to Boat Harbour, where Mawson moored the Aurora in January 1912. Until last week, no-one knew if the "fast ice" was safe enough to get across with surface vehicles.

"Getting to the huts was something we've worked so hard for these last two years," said Turney. "It's amazing how all those years of preparation tunnel down to this few days where, if the conditions aren't in your favour, it's game over."

Their five-hour journey was riddled with stops as the vehicles got trapped in sludgy pools of melted ice and snow, but they made it to Mawson's huts by lunchtime, staying on site for 12 hours before making the arduous journey back to the ship. A second group made the same journey on Friday.

Turney said he felt not only a sense of relief at reaching the huts, but also a good deal of emotion at reaching the place that was the inspiration behind his own AAE. "This is a place I've lived at in my dreams for the last two years," he said. "It's almost as close to a holy ground as you could get on this trip."

Six scientists made it to Cape Denison over two days and they used their limited time, among other projects, to record how the presence of large swaths of fast ice caused by B09B, where there should be open ocean, has caused a collapse in the ecology of the local seabed. They also found that the many nearby colonies of Adelie penguins have suffered – these birds need access to the ocean to feed and the nearest shoreline is more than 60km away, because of the fast ice.

The scientist were also accompanied by a conservation team that carried out maintenance work on Mawson's huts themselves, century-old buildings that have become icons of the original expedition and which have not been properly visited since 2011, due to the problems with access.

Turney's team had hoped to repeat more of Mawson's measurements at Cape Denison but the difficulty in getting to the site meant they had to cut down their research programme.

Glaciologist Chris Fogwill, for example, tried to drill an ice core above Cape Denison – these cores are filled with ancient air bubbles that provide a record of the Earth's atmosphere over several thousand years – but the conditions on the day were not quite right to do it properly. "The problem was that it was so warm at the time, the water was flowing over the surface and re-freezing when you were drilling down any depth," said Turney. "I'd love to go at a different time of year – who knows what climate record is in that, that's got huge potential."

Beyond the Antarctic coastline, scientists on board the Shokalskiy have been busy with research that has, so far, included everything from measuring the temperature and salinity of the surface layers of the Southern Ocean to counting its bird populations and deploying robotic probes and buoys to map long-term information about ocean currents and temperatures.

On the two-week return journey to New Zealand, the scientists will continue their study of the ocean and, at the half-way point of the return journey at the sub-Antarctic Macquarie Island, Turney will also drill a metre into the sea bed to extract a core of the sediments there, which should provide details of how the sea environment has changed over the past few thousand years. His teams will also measure sea temperatures in the area, work that Mawson did on the original AAE but which has only been repeated once since, in the 1950s.

In just over a week, the modern AAE will pull into port in Bluff, New Zealand. Meanwhile, Turney knows that the carefully-made plans for the final few days of work could change at any second, depending on the conditions at sea or on the lands they visit. But that is just a consequence of working in the Antarctic, according to Turney.

"You've got to treat the land and ice and environment you're in with an enormous amount of respect, which you don't have to do when you're back home in the civilised world, where you can get away with almost being unaware of your environment and you'll survive," he said.

"Here you can't do that, you have to be so careful all the time. In a way, that's also lovely – it's not often that you just make observations about the landscape you're in and understand that landscape. It's an incredible privilege and honour to be out in these environments."